Austenitic alloy containing boron and processes for manufacturing thesame

ABSTRACT

An austenitic ferrous base alloy characterized by improved stability at high temperatures, and by improved resistance to sensitization and to corrosion, cracking, and general deterioration. The austenite is an iron-chromium-nickel alloy containing from 0.01 to 0.2 percent boron, less than 0.009 percent carbon, and substantially free from nitrogen and oxygen. The boron is a solute in the austenite. The process of manufacture is to form a melt of iron, chromium, nickel and other additives, and then add a ferro-boron alloy having a melting point well below that of the base alloy. The process is carried out in a vacuum furnace in order to substantially eliminate carbon, nitrogen, and oxygen from the alloy.

United States Patent [191 Kanter Aug. 28, 1973 AUSTENITIC ALLOY CONTAINING BORON,

AND PROCESSES FOR MANUFACTURING THE SAME l [76] Inventor: Jerome J. Kanter, Palos Park, 111.

[22] Filed: Dec. 14, 1970 [21] Appl. No.: 98,101

[52] U.S. C1. 75/129, 75/49, 75/128 F [51] 'Int. Cl. C220 39/20, C22c 39/54 [58] Field of Search 75/128 R, 49, 129,

[56] References Cited UNITED STATES PATENTS 3,065,067 11/1962 Aggen 75/124 3,065,068 11/1962 Dyrkacz et al.... 75/124 3,199,978 8/1965 Brown et a1. 75/128 3,210,213 10/1965 Cotter et al.... 117/205 3,212,884 10/1965 Soler 75/124 2,815,273 12/1957 Moore... 75/128 R X 3,107,997 10/1963 Kozlik 75/124 -Assistant Examiner.l. E. Legru Attorney-Hume, Clement, Hume 84 Lee [57] 1 ABSTRACT An austenitic ferrous base alloy characterized by improved stability at high temperatures, and by improved resistance to sensitization and to corrosion. cracking, and general deterioration. The austenite is an ironchromium-nickel alloy containing from 0.01 to 0.2 percent boron, less than 0.009 percent carbon, and substantially free from nitrogen and oxygen. The boron is a solute in the austenite. The process of manufacture is to form a melt of iron, chromium, nickel and other additives, and then add a ferro-boron alloy having a melting point well below that of the base alloy. The process is carried out in a vacuum furnace in order to substantially eliminate carbon, nitrogen, and oxygen from the alloy.

8 Claims, No Drawings 12/1964 Copson 751128 R AUSTENITIC ALLOY CONTAINING BORON, AND PROCESSES FOR MANUFACTURING THE SAME FIELD OF THE INVENTION The invention relates to improvements in austenitic iron base alloys for use at elevated temperatures, and to processes for manufacturing the same, and more particularly to austenitic alloys containing boron that demonstrate improved high temperature stability.

BACKGROUND TO THE INVENTION Austenitic iron base alloys have found many uses. They are used in articles in which strength, toughness, and resistance to corrosion are required. At elevated temperatures, however, for example. at-temperatures above about 1000F, austenitic stainless steels loose their stability, become sensitized, and are made susceptible to corrosion, cracking, and general deterioration.

During recent years much attention has been directed to the problem of developing stainless steel alloys having high temperature stability. The need for alloys that will withstand corrosion, cracking and deterioration under service conditions involving high temperatures has become highly important in the utility power industry and in the component parts of jet engines. Alloys for use in such high temperature applications demand certain criteria, the most predominant of which concerns rupture strength and rupture ductility, sensitization resistance, oxidation and corrosion resistance, high strength, hardness, and toughness at elevated temperatures.

It is now commonly recognized in the power utility industry that the austenitic alloys should not be exposed to temperatures above about l,000"F in fabrication or service for substantial periods of time withoutadverse deterioration and sensitization. The temperature of 1,05 F is regarded as the austenite barrier.

Sensitization of the alloy as used herein refers to the phenomena that occurs when the alloy is heated to elevated temperatures for a period of time. Sensitization causes adverse effects: cracking, corrosion, and general deterioration of the metal. Sensitization is a time-temperature eifect, and use of the austenitic alloy in service for extended periods of time at about 1,000F or at higher temperatures for shorter periods of time results in corrosion and cracking.

Various additives for austenitic alloys have been advocated to prevent sensitization, but none have proved adequate to the demands of elevated temperature service.

Boron has been incorporated in some prior alloys as a precipitation hardner. While precipitation hardened boron alloys may improve the ductility of otherwise brittle alloys, there is little or no improvement in the high temperature stability, or sensitization resistance, the latter of which is a property critically necessary for uses of the alloy encountering temperatures above 1,000F for prolonged periods of time.

SUMMARY OF THE INVENTION It is an object, therefore, of the present invention to provide austenitic iron base alloys having high temperature stability, particularly to austenitic iron base alloys having improved stability or sensitization resistance, at temperatures in the range above 1,000'F, and to provide processes for the manufacturing of the same.

Another object of the invention is to provide austenitic alloys characterized by improved resistance to corrosion and cracking.

Still another object of the invention is to provide an austenitic iron base alloy having improved stability at elevated temperatures.

Yet another object of the invention is to provide an austenitic iron base alloy which has improved resistance to corrosion in the manufacture of parts for gas and steam generators, nuclear reactor systems, turbines, jet engines for aircraft, piping systems, and the like, which operate at temperatures above 1000F.

Still yet another object of the invention is to provide a ferrous base austenitic alloy containing boron in which the precipitation of boron is avoided or substantially reduced.

Other objectives of the invention will be apparent from the following description and appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS It is a discovery of the present invention that an austenitic alloy having high temperature stability can be produced by substantially eliminating the interstitial elements, carbon, nitrogen, and oxygen, and by incorporating boron in lieu thereof.

Briefly stated, the austenitic alloys of the invention are iron-chromium-nickel alloys which contain from about 0.01 percent by weight to about 0.2 percent by weight boron, and which are substantially free from carbon, nitrogen and oxygen. In the alloys of the invention, for instance, the carbon is eliminated as much as practicable, and ordinarily will be less than 0.009 percent by weight. In the alloys of the invention the boron is a solid solute in the austenite.

The austenitic alloys of the invention, possessing the improved properties at elevated temperatures, comprise broadly from about 12 percent to about 30 percent by weight chromium, from about 8 percent to about 20 percent by weight nickel, from about 0.01 percent to about 0.2 percent by weight boron, 0 percent to about 2.5 percent by weight manganese, 0 percent to about 1.5 percent silicon, less than 0.009 percent by weight carbon, and the balance iron with incidental impurities. Such alloys are substantially free from carbon, nitrogen and oxygen, and from the precipitated forms of boron.

An austenitic alloy of the invention having particular utility has a composition within the range between 18 percent to 20 percent chromium, 8 percent to 12 percent nickel, 0.03 percent to 0.15 percent boron, 0 percent to 2 percent manganese, 0 percent to 1 percent silicon, less than 0.009 percent carbon, the balance iron with incidental impurities, and substantially free from nitrogen and oxygen.

Each of the alloying elements present within the broad range as set forth above performs a specific function.

The austenitic alloy of the invention depends upon the boron element locked within the lattice structure. Precipitation of the boron or migration of the boron through the alloy at high temperatures is to be avoided, and for this reason elements that react with the boron at elevated temperatures, particularly carbon and nitrogen, and also oxygen, or any other elements that might interfer with the solution of boron, are eliminated as far as practicable. In the ferrous base austenite of the invention, the boron remains soluble in the alloy, EXAMPLE VII and during preparation and use efforts are made to Boron 003F059, maintain the boron as a solute 1n the austenite. Chromium 17-19% The boron content usually is within the range from nickel g 0.01 percent to 0.2 percent, and preferably in the range 5 ggz 1% from 0.03 percent to 0.15 percent by weight, in the ab- Phosphorus 0.045 max sence of nitrides, carbides, or oxides thereof. remand The boron employed in the alloy may be the naturally EXAMPLE vm occuring element (B) having an atomic weight of 10.82. For some applications, however, it may be adgzg' i 'g vantageous to employ the isotope of boron (18 having Nickel 342% an atomic weight of about 1 l. The isotope B provides ggfiggg f max several advantages. It has less tendancy to migrate in on remainder the alloy than the naturally occuring element. Also, in

EXAMPLE IX,

connection with atomic energy plants, the isotope may be more desirable since it has a lower nuclear capture Boron I 0 012% cross-section than natural boron. ghrpn uum I Chromium is the predominant element for providing 'g g 2% max COI'IOSIOI] resistance and oxidation resistance to the Silicon 1% rnax alloy at elevated temperatures. Chromium also enters 20 remand the solid solution and materially contributes to the EXAMPLE X strength of the matrix when it is present within the broad range. Chromium usually is in amounts less than gf zg' l I gig 58 22 percent by weight. Chromium in excess of 22 per- Nickel 19-22% cent by weight tends to counteract fpll austenization, 25 a: max promoting partially ferrite structures, comprising high on remainder temperature stability. Although chromium is a strong ferrite forming element, when it is in solution at ele- EXAMPLE XI vated temperatures it reacts to retard structural m 091410896 changes and thus tends to stabilize the alloy. 30 23:22:: Nickel is the predominant austenite forming element Manganese 2% max and acts in cooperation with the chromium to provide Silicon l.5% max lron remainder oxidation and corrosion resistance. Nickel is usually present in amounts from 8 percent to about 15 percent weight, but higher amounts may be employed.

Manganese, if present, functions not only as a scav enger, but may increase the rupture ductility of the alloy, and may confer hot workability to the alloy. Silicon, if present, functions not only as a scavenger, but may confer strength to the alloy, and also may contrib- The alloys are P'Qp y double vacuum meltingute to oxidation resistance Vacuum 1s first applied to the components before melt- While other additives may be beneficial, the alloy is i to remove oxygen and gf and also to the mellsubstantially free of elements, or impurities, that intermg procedures- T vacuum be Pressures fer with the solubility of boron in the austenite. For exbelow atmosphenc, and ordmanly at Pressures,

The alloys 1 to Xl above are substantially free from carbon, nitrogen, and oxygen, and preferably the total amount of said elements is as low as practically possible, and most desirably, the combined total of all of said elements does not exceed 0.02percent by weight.

ample, elements that react with boron to form insoluble and as low as 100 microns and below- The p compounds in the austenite are to be avoided tures of melting may be in the range from 2,500F to The presence of nitrogen and/or carbon in the alloy 3,0000}:- has been discovered to interfer with the solubility of the The alloy of the present invention is prepared in a boron in the austenite therby reducing the stability of vacuum induction furnace. The vacuum furnace is necthe alloy at elevated temperatures. The alloy of the inessary for the out gassing of the elements carbon, nitrovention, therefore, is substantially free from the presgen, and oxygen from the melt, and to eliminate these ence of elements nitrogen and carbon. A total content particular elements as far as practicable. One proceof less than 0.01 percent by weight of these elements dure is to first melt a mixture of iron and nickel compowill be effective for this purpose. nents, and, thereafter ferro-chromium, or pure chro- Specific examples of austenitic ferrous base alloys of mium, or other suitable alloys thereof, may be added in the invention are set forth in the following table in the desired proportions along with any other compowhich the percentages are by weight: nents, for example, manganese and/or silicon. The melt TABLE I.AUSTENITIC ALLOY EXAMPLES I iltlplt I II III IV I Boron .I)% 08 Chromium I ickvl 1 Mnngum 2.0% nu 2.0% 1uax .2 )1 max Phosphor 0.450} max 0.45% max... 0.45% max 0 0.45% max. Silicon l.0,T,'- nmx I.0%n1ax 1.0% max 1.5% max, 1.0% max. 1.091 max. Carbon 0.000% max. 0.000% max 0.000); nmx 0.000% nmx 0.000%1uox-.. 0.000% max.

In the following examples are given exemplary useful should be thoroughly stirred, if possible, by electroaustenitic alloys embodying the invention: magnetic coils to assure uniform combination of all the components of the melt. Shortly before the melt is to be poured, the desired amount of ferro-boron alloy is added. The chromium and silicon additions precede the boron addition in order to eliminate any nuclei that might cause precipitation of the boron. Due to its high melting point, boron is added as a ferro-boron alloy having a melting point substantially below the melting point of the base alloy to avoid nucleation of refractory chromium borides. The ferro-boron alloys containing less than percent by weight of boron are satisfactory for this purpose. The melt is stirred, and then may be teemed into ingots.

In order to produce the finished articles, such as turbine blades, rotor discs, tubing, and the like, the ingots may be forged or wrought after heating them to temperatures between l,500F and 2,200F. Heating at these elevated temperatures should be performed under a protective atmosphere of inert gases, such as helium, argon, and neon, or mixtures thereof, in order to prevent oxygen and nitrogen from entering the alloy and reacting with the boron. In some instances, the articles may be cast, for instance, by precision casting, or shell molding, procedures from the melt without forging or working to the desired shape. The cast members may be ground or machined to the desired dimensions and configurations.

The novel austenitic alloys may be used for many purposes, but it has particular use under service conditions employing high temperatures.

In connection with the use or working of the austenitic alloys of the invention at elevated temperatures, for example, during welding, a protective atmosphere of inert gases such as helium, argon, neon, or mixtures thereof, may be used in order to prevent deterioration. Other inert gases may be employed which will not diffuse into the alloy and react with the boron element.

Although I do not wish to be bound by my theories, the following explanation is furnished to further describe the invention. The austenitic stainless steel alloys which are currently the subject of much investigation because of the sensitization problem is an acute form of the phenomena to which all iron based alloys are more or less susceptible upon being heated to elevated temperatures. Sensitization is a phase change occasioned by the diffusion or migration of the interstitial elements in the alloy, unlocking the phase transformation from austenite to ferrite. For example, it is well known as to the 300 series austenitic stainless steels, that in order to put them in thebest condition to resist corrosion and cracking, they should be heated to 2,100F to solutionize the interstitial elements, and then rapidly cooled. Subsequent heating, as may occur during fabrication and service, tends to cause the metal to revert to the sensitized state. The reversion to the sensitized state is a time-temperature dependent effect, and the metastable"austenite sooner or later reverts in some measure to ferrite and the sensitized condition.

The element boron serves as an interstitial alloying element. It readily enters into the solution of the alloy and is locked within the lattice of the alloy. It is less susceptible to migration or diffusion than the other commonly employed interstitial elements, such as carbon and nitrogen.

Although prior workers in the field were familiar with boron alloys of various types, the alloys of the invention have not been recognized, because of several reasons. The reaction of boron with various elements present in the alloy, or in the atmosphere during the production of the alloy, such as carbon, nitrogen, and oxygen, and- /or the adverse effects produced thereby was not appreciated. The discovery of the high temperature sta- 5 bility of austenitic alloys containing boron of the invention is associated with the discovery that the boron heretofor has been permitted to react with other elements to form undesirable components, the failure to recognize the significance of boron when properly employed in reducing the diffision and migration of the interstitial elements upon heating, and the failure to understand the role of the interstitial elements to the sensitization problem. For example, addition of boron to a nitrogen containing alloy would nucleate a compound with a melting point around 5,400F, and boron nitride thus formed cannot be solutionized in the alloy by the customary heat treatment which comprises heating to 2,100F and then quenching. As another example, reaction of carbon with boron'to form boron carbide nucleates a compound having a melting point at about loys of the present invention are not to be confused with stainless steel, since steel by definition is a form of iron containing carbon. By contrast, the alloys of the present invention eliminate carbon as far as practicable.

Other modes for applying the principles of the invention may be employed, change being made in regard to details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I claim:

1. In a process for preparing ferrous base alloys characterized by stability at elevated temperatures, the steps comprising, preparing a melt having a composition within the range between about 12 percent to about 30 percent chromium, about 8 percent to about 20 percent nickel, 0 percent to about 2.5 percent manganese, 0 percent to 1.5 percent silicon, less than 0.009 percent carbon, and the balance iron with incidental impurities, maintaining said melt under vacuum pressures, adding to said melt sufficient ferro-boron alloy to introduce 0.01 percent to 0.2 percent by weight boron, said ferro-boron alloy having a melting point substantially below the melt temperature and containing less than 10 percent by weight boron, and teeming said melt into a mold.

2. A ferrous base alloy characterized by high temperature stability and by being substantially completely austenitic, consisting essentially of from about 12 per- 5 cent to about 30 percent by weight chromium, from about 8 percent to about 20 percent by weight nickel, from about 0.01 percent to about 0.2 percent boron, 0 percent to about 2.5 percent by weight manganese, 0 percent to about 1.5 percent by weight silicon, less than 0.009 percent by weight carbon, and in which the total amount of carbon and nitrogen is less than 0.01 percent by weight, and the balance iron with incidental impurities that do not interfere with the solubility of boron, said boron in the form of a solid solute in the austenite, said alloy substantially free from carbon, nitrogen, and oxygen in total amounts that will react with the boron and adversely affect the alloy, and said alloy substantially free from the precipitated forms of boron.

iron with incidental impurities that do not interfere with thesolubility of boron, said alloy substantially free from carbon, nitrogen, and oxygen, in total amounts that will react with the boron and adversely affect the alloy.

4. An austenitic ferrous base alloy characterized by high-temperature stability having essentially the following composition:

Boron 003-0. Chromium 17-19% Nickel 840% Manganese 2% max Silicon 1% max Phosphorus 0.045% max Carbon 0.009% max lron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen in total amounts that will react with the boron and adversely affect the alloy.

5. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition:

Boron 0.024108% Chromium 18-20% Nickel 842% Manganese 2% max Silicon 1% max Carbon 0.009% max lron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen, intotal amounts that will react with the boron and adversely affect the alloy.

6. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition:

Boron 0.012% Chromium 17-19% Nickel 10-13% Manganese 2% max Silicon 1% max Carbon 0.009% max lron remainder said boron in the form of a solid solute in the austenite, and said-alloy substantially free from carbon, nitrogen and oxygen in total amounts that will react with the boron and adversely affect the alloy.

7. An austenitic-ferrous base alloy characterized by high temperature stability having essentially the following composition: I

Boron Chromium 22-24% Nickel 19-22% Manganese 2% max Silicon 1% max Carbon 0.009% max lron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen, in total amounts that will react with the boron and adversely affect the alloy.

8. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition:

Boron Chromium 24-26% Nickel 19-22% Manganese 2% max Silicon 1.5% max Carbon 0.009% max lron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen in total amounts that will react with the boron and adversely affect the alloy. 

2. A ferrous base alloy characterized by high temperature stability and by being substantially completely austenitic, consisting essentially of from about 12 percent to about 30 percent by weight cHromium, from about 8 percent to about 20 percent by weight nickel, from about 0.01 percent to about 0.2 percent boron, 0 percent to about 2.5 percent by weight manganese, 0 percent to about 1.5 percent by weight silicon, less than 0.009 percent by weight carbon, and in which the total amount of carbon and nitrogen is less than 0.01 percent by weight, and the balance iron with incidental impurities that do not interfere with the solubility of boron, said boron in the form of a solid solute in the austenite, said alloy substantially free from carbon, nitrogen, and oxygen in total amounts that will react with the boron and adversely affect the alloy, and said alloy substantially free from the precipitated forms of boron.
 3. An austenitic iron-base alloy having a composition within the range between from about 12 percent to about 30 percent chromium, from about 8 percent to about 20 percent nickel, from about 0.03 percent to about 0.15 percent boron, from 0 percent to about 2.5 percent manganese, from 0 percent to about 1.5 percent silicon, less than 0.009 percent by weight carbon, and in which the total amount of carbon and nitrogen is less than 0.01 percent by weight, said boron in the form of a solid solute in the austenite, and the balance iron with incidental impurities that do not interfere with the solubility of boron, said alloy substantially free from carbon, nitrogen, and oxygen, in total amounts that will react with the boron and adversely affect the alloy.
 4. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition: Boron 0.03-0.15% Chromium 17-19% Nickel 8-10% Manganese 2% max Silicon 1% max Phosphorus 0.045% max Carbon 0.009% max Iron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen in total amounts that will react with the boron and adversely affect the alloy.
 5. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition: Boron 0.02-0.08% Chromium 18-20% Nickel 8-12% Manganese 2% max Silicon 1% max Carbon 0.009% max Iron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen, in total amounts that will react with the boron and adversely affect the alloy.
 6. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition: Boron 0.012% Chromium 17-19% Nickel 10-13% Manganese 2% max Silicon 1% max Carbon 0.009% max Iron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen in total amounts that will react with the boron and adversely affect the alloy.
 7. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition: Boron 0.01-0.08% Chromium 22-24% Nickel 19-22% Manganese 2% max Silicon 1% max Carbon 0.009% max Iron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen, in total amounts that will react with the boron and adversely affect the alloy.
 8. An austenitic ferrous base alloy characterized by high temperature stability having essentially the following composition: Boron 0.01-0.08% Chromium 24-26% Nickel 19-22% Manganese 2% max Silicon 1.5% max Carbon 0.009% max Iron remainder said boron in the form of a solid solute in the austenite, and said alloy substantially free from carbon, nitrogen and oxygen in total amounts that will react with the boron and adversely affect the alloy. 